/Engine/Source/Runtime/AudioMixer/Public/AudioMixerSubmix.h
void MixBufferDownToMono ( const [FAlignedFloatBuffer](API\Runtime\SignalProcessing\DSP\FAlignedFloatBuffer) & InBuffer, int32 NumInputChannels, [FAlignedFloatBuffer](API\Runtime\SignalProcessing\DSP\FAlignedFloatBuffer) & OutBuffer )